US4672359A - Superconducting analog-to-digital converter and digital magnetometer and related method for its use - Google Patents

Superconducting analog-to-digital converter and digital magnetometer and related method for its use Download PDF

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US4672359A
US4672359A US06/797,314 US79731485A US4672359A US 4672359 A US4672359 A US 4672359A US 79731485 A US79731485 A US 79731485A US 4672359 A US4672359 A US 4672359A
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signal
squid
analog
current
operating point
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Arnold H. Silver
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Northrop Grumman Corp
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TRW Inc
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Priority to DE8686306033T priority patent/DE3684558D1/de
Priority to EP86306033A priority patent/EP0222470B1/en
Priority to JP61269477A priority patent/JPS62117419A/ja
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Priority to JP4076526A priority patent/JP2711043B2/ja
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    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
    • G01R33/00—Arrangements or instruments for measuring magnetic variables
    • G01R33/02—Measuring direction or magnitude of magnetic fields or magnetic flux
    • G01R33/035—Measuring direction or magnitude of magnetic fields or magnetic flux using superconductive devices
    • G01R33/0354—SQUIDS
    • G01R33/0356—SQUIDS with flux feedback
    • H—ELECTRICITY
    • H03—ELECTRONIC CIRCUITRY
    • H03M—CODING; DECODING; CODE CONVERSION IN GENERAL
    • H03M1/00—Analogue/digital conversion; Digital/analogue conversion
    • H03M1/12—Analogue/digital converters
    • H03M1/48—Servo-type converters
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y10—TECHNICAL SUBJECTS COVERED BY FORMER USPC
    • Y10S—TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y10S505/00—Superconductor technology: apparatus, material, process
    • Y10S505/825—Apparatus per se, device per se, or process of making or operating same
    • Y10S505/842—Measuring and testing
    • Y10S505/843—Electrical
    • Y10S505/845—Magnetometer
    • Y10S505/846—Magnetometer using superconductive quantum interference device, i.e. squid

Definitions

  • This invention relates generally to high-performance magnetometers and analog-to-digital (A/D) converters and, more particularly, to techniques for A/D conversion employing superconducting Josephson junction devices.
  • Performance criteria include sensitivity, dynamic range, and sampling rate.
  • High sensitivity instruments, such as magnetometers and gradiometers, employing superconducting devices typically suffer from limitations in their signal bandwidth, their ability to handle rapid slew rates of incoming signals, and their dynamic range. As will be further explained, the present invention overcomes these disadvantages.
  • a Josephson junction has a current-voltage characteristic that includes a region in which the current increases rapidly from zero, with practically no corresponding increase in voltage across the device.
  • a SQUID is a circuit including one or more Josephson junctions and one or more inductive loads.
  • a single-junction SQUID includes a Josephson junction connected across an inductance. If a current is injected into one end of the inductance and the other end is grounded, the resulting characteristics provide the basis for A/D conversion, as explained in detail in the Hurrell et al. paper.
  • the most pertinent property of the single-junction SQUID is the relationship between the magnetic flux in the SQUID and the value of the injected current.
  • This flux-current relationship is a periodic function and, depending on the circuit parameters chosen, a multi-valued function.
  • the most significant aspect of the relationship is that the flux changes by a small quantum whenever the current increases by a small and precisely repeatable increment. This quantum of flux gives rise to a small but measurable voltage pulse across the junction. When the current is decreased, a flux quantum of opposite polarity is produced for each precise decrement of current, and a corresponding voltage pulse of opposite polarity is produced across the junction.
  • This property of the single-quantum SQUID forms the basis for the A/D converter described in the Hurrell et al. paper.
  • a signal to be converted from analog to digital form is introduced into the single-junction SQUID as a varying current. Each time the current decreases by a predetermined increment, a measurable voltage pulse is generated across the junction. In this manner, the single-junction SQUID functions as a quantizer. The resultant pulses are then detected and counted in one or more counters.
  • the principal advantage of the arrangement is its near perfect linearity. Another advantage is its sensitivity.
  • the current increment which determines the resolution, can be made extremely small. The flux quantum is only 2.07 ⁇ 10 -15 weber, and the current increment is given by this value divided by the value of the load inductance (measured in henries).
  • the basic mode of operation described suffers from two important limitations. One is that, if the input signal slews rapidly in value, it may not be possible to detect each of the flux transitions that occur. The other is that the sensitivity of the device in its basic form is limited to one flux quantum. Attempts to obtain sensitivity less than one quantum have resulted in a very much limited dynamic range.
  • One attempt to provide finer sensitivity involves the use of a feedback signal to maintain a stable operating point on the flux-current curve. Basically, this is accomplished by applying a high-frequency "dither" current to the Josephson device and sensing any resulting output signals at the dither frequency.
  • a feedback loop is used to drive the high-frequency output component to zero, a condition that will be obtained only if the operating point on the flux-current curve is at the desired location.
  • the feedback signal provides an indication of the degree to which the input signal being converted differs from a signal that would have left the operating point undisturbed.
  • this arrangement still suffers from limitations in slew rate and dynamic range.
  • the approach requires the use of analog amplifiers or integrators, which may detract from the accuracy of the device.
  • U.S. Pat. No. 3,983,419 to Fang discloses a SQUID that can be used as either a sample-and-hold device or as a pulse generator coupled to an analog signal. The resulting signals can then be used in an A/D converter, the details of which are not disclosed.
  • U.S. Pat. No. 3,458,735 to Fiske discloses a system for employing a plurality of Josephson junctions for A/D conversion.
  • U.S. Pat. No. 4,315,255 to Harris et al. discloses an A/D converter using multiple SQUID's of the voltage latching type.
  • the ideal converter should have high sensitivity and speed, but also a large dynamic range and the ability to accept input signals with rapid slew rates.
  • the present invention is directed to these ends.
  • the present invention resides in a superconducting magnetometer or analog-to-digital converter having a sensitivity equivalent to a small fraction of a single flux quantum, but also having a large dynamic range and the ability to handle rapid input signal slew rates.
  • the device of the invention includes a double-junction superconducting quantum interference device (SQUID) having first and second Josephson junctions and a center-tapped load inductance, means for applying a constant gate current to the center of the load inductance, and means for applying a bidirectionally varying analog signal current to the load inductance. Positive incremental changes in the load inductance current result in the generation of voltage pulses across the first junction, and negative incremental changes in the load inductance current result in the generation of voltage pulses across the second junction.
  • SQUID double-junction superconducting quantum interference device
  • the device of the invention includes means for applying a correction current signal to the load inductance to maintain the SQUID at a desired operating point in its flux-current characteristic, and means for applying a high-frequency dither signal current to the load inductance such that, when the correction current is maintaining the SQUID at its desired operating point, each cycle of the dither current signal produces an output pulse from each of the junctions, means for generating from the output pulses a digital value equivalent to the analog input signal, and means for generating the correction current from the digital value.
  • the correction current is indicative of the amount by which the analog signal would, but for the application of the correction current, move the operating point away from its desired location.
  • the correction signal therefore maintains the operating point at the desired location, and in so doing generates a digital signal indicative of the analog signal as a fraction of a single quantum value.
  • the range of the device is not limited to one quantum value, since the means for generating a digital value also includes means for maintaining a count equivalent to the integral number of flux quanta by which the analog signal has changed. This count is also associated with a quantized change in the quiescent operating point, to the next periodically occurring and equivalent operating point.
  • the means for generating a digital signal includes means for detecting voltage pulses from the SQUID, a feedback register, an integer register, and control means for adjusting the feedback register and the integer register in response to the output pulses detected from the SQUID.
  • the feedback register contains a count indicative of a fraction of a single flux quantum.
  • the control means increments or decrements the feedback register.
  • the feedback register is selected to overflow when the analog current changes by an amount equivalent to one flux quantum.
  • the feedback register is cleared to zero and the the integer register is incremented by one. This effectively moves the operating point to the next suitable location on the curve, and keeps count of the number of flux quanta in the integer register.
  • the feedback register is connected to a digital-to-analog converter, the output of which is the correction current fed back to the SQUID.
  • the invention comprises the steps of applying the analog signal to the SQUID, applying a gate current to the SQUID to produce a characteristic flux-current curve with periodic hysteresis loops, and applying a high-frequency dither signal to the SQUID to produce two output pulses per cycle of the dither signal, so long as the operating point is maintained at a quiescent point centrally located with respect to one of the hysteresis loops.
  • the method further includes the steps of detecting the presence or absence of output pulses from the SQUID junctions, determining from the detected pulses whether the analog signal applied to the SQUID is tending to move the operating point along the SQUID characteristic curve, incrementing a feedback register if movement is sufficient to result in non-detection of one of the pulses normally expected in a cycle, converting the value stored in the feedback register into an analog correction signal, and applying the correction signal to the SQUID to compensate for the detected movement in the operating point.
  • the method may further include the steps of determining from the detected pulses whether the analog signal has slewed from its previously detected value by more than at least one full quantum, and incrementing an integer register to record the number of full flux quanta by which the analog signal has changed.
  • Another aspect of the method invention includes the steps of detecting when the feedback register overflows, which is indicative of a full quantum of movement of the analog signal, and incrementing the integer register and simultaneously clearing the feedback register when the overflow is detected.
  • the feedback register contains a number of bits of the desired digital result, at the least significant end of the result.
  • the integer register contains the remainder of the digital result, at the most significant end.
  • the control means of the invention takes care of rapidly slewing input signals, since these can also be detected in the SQUID. If a signal slews so rapidly that two transitions are detected in the same direction, this indicates that the operating point of the device has effectively jumped to the next multi-valued location on the characteristic curve.
  • the control means increments or decrements the integer register to take care of this situation, so that even signals that slew more rapidly than the dither signal rate will be correctly converted by the device of the invention.
  • the present invention represents a significant advance in the field of high-performance analog-to-digital converters.
  • the converter of the invention achieves an extremely high sensitivity, but without loss of dynamic range and with the ability to handle high input signal slew rates.
  • Other aspects and advantages of the invention will become apparent from the following more detailed description, taken in conjunction with the accompanying drawings.
  • FIG. 1 is a block diagram of an analog-to-digital converter in accordance with the invention
  • FIG. 2 is a graph of the characteristic flux-current curve for a SQUID as used in the converter of the invention
  • FIG. 3 is a schematic diagram of a digital-to-analog converter for use in the analog-to-digital converter of FIG. 1.
  • the present invention is concerned with superconducting analog-to-digital (A/D) converters.
  • A/D analog-to-digital
  • high-performance A/D converters of this type have suffered from limitations in their sensitivity, dynamic range, and ability to handle input signals having rapid high slew rates.
  • a double-junction superconducting quantum interference device (SQUID) is operated at a desired point in its flux-current characteristic, by applying a correction signal derived from a component of the converted analog signal. As the analog signal changes, the correction signal also changes to maintain the desired operating point.
  • SQUID superconducting quantum interference device
  • the converter of the invention includes, a double-junction SQUID, indicated generally by reference numeral 10, a binary pulse detector 12, control logic 14, a feedback register 16, an integer register 18, and a digital-to-analog converter (DAC) 20.
  • the SQUID 10 has two Josephson junctions 22 and 24, connected back to back, with a common grounded terminal, and a center-tapped load inductor 26, with a gate current source 28 applied between ground and the inductor center tap. Current is coupled to the load inductor 26 through three input windings: a dither input winding 30, a correction current winding 32 and an analog input winding 34. It will be appreciated that an alternative approach would be to use a single input winding and to combine three input signals with an appropriate connection of resistors to the winding.
  • An analog input signal is applied to the analog input winding 34, and a high-frequency clock signal is applied from a clock generator 36 to the dither input winding 30.
  • the flux-current characteristic of a SQUID device is a periodically varying function like the one shown in FIG. 2. At periodic intervals, the curve exhibits a hysteresis effect and, with an appropriate gate current, assumes multiple flux values.
  • the dither signal from the clock generator 36 is selected to have an amplitude just sufficient to cycle the flux and current through the closed loop indicated by numeral 40. At each sudden transition in magnetic flux, at the points 42 and 44, an output pulse is generated by the SQUID.
  • the output pulse is generated on one of the output lines 46 and 48 from the SQUID, and for the negative going transition the pulse is generated on the other of the two output lines. Operation of the double-junction SQUID in this manner is described in the cross-referenced patent application. So long as the operating point of the device is maintained at a quiescent operating point 50, the device will continue to generate alternate output pulses on the two output lines 46 and 48.
  • the current position on the curve can be determined from an analysis of the SQUID output signals. Detecting these signals, which will be referred to as the A and B signals, is accomplished in the binary pulse detector 12.
  • the flip-flops are enabled by the clock signals, as indicated by line 60 from the clock generator 36.
  • the control logic 14 monitors the state of the flip-flops in the pulse detector 12, as indicated by line 62, and generates control signals on line 64 to the feedback register 16 and on line 66 to the integer register 18, as will now be explained in further detail.
  • the control logic 14 If an A signal is detected, but not a B signal, this indicates that the operating point has moved away from the hysteresis loop, i.e. that the current has increased by some amount less than the equivalent of a full flux quantum.
  • the action then taken by the control logic 14 is to preset the output to the feedback register to a value of +1.
  • the analog signal will either continue to increase, stay at its new level, or return to its previous level. If it returns to its previous level, near the original operating point, A and B signals will be detected again and no action will be taken by the control logic. However, if the signal continues to increase or stays at its new level, there may be one or more clock cycles during which neither the A nor the B signal is detected. If this is the case, the control logic 14 outputs the preset value to the feedback register. In the example given above, a +1 value would be output to the feedback register, which would be incremented by one.
  • the key to the invention is that the value stored in the feedback register is converted to an analog value and fed back to the SQUID as a correction current. Conversion to analog form takes place in the DAC 20, the output of which is connected through a feedback resistor 68 to the correction current winding 32.
  • control logic 14 There are two other possibilities for the control logic 14. If two or more successive A signals are detected, this indicates that the input signal has slewed from the original operating point to another operating point in one clock cycle. Rather than attempting to correct for this state of affairs, the control logic instead outputs a +1 value to the integer register 18. Similarly, the detection of two successive B signals in the same clock cycle results in the transmission of a -1 to the integer register.
  • control logic performs the following functions.
  • the overlined symbols A and B mean “not A” and “not B”, respectively.
  • the "action taken" in the first line of the following table assumes that the SQUID is initially in the negative state, indicated by the lower curve (-) in FIG. 2. If the SQUID is initialized to the positive state (+), the first line of the table should indicate a preset output of +1 instead of -1.
  • the feedback register contains at all times a digital quantity indicative of the amount by which the analog signal differs from a current at the desired operating point.
  • the feedback register has a full-scale value equivalent to the span between adjacent operating points in the characteristic curve of the device, and also carries a sign indication to show whether the current is tending to drive the operating point above or below the desired operating point.
  • the feedback register and the integer register together define a digital value equivalent to the input analog current. More accurately, they define the value of the input signal relative to a given starting point. For any particular setting of the integer register, the feedback register applies corrections over a range of plus and minus one full span between adjacent operating points on the curve.
  • the principal advantage of the invention over other SQUID A/D converters is that it provides extremely high sensitivity but a large dynamic range and the ability to handle rapid slew rates. Also, because the device is implemented as a monolithic cryogenic structure it enjoys a practically complete absence of noise problems. It will by now be understood that the sensitivity of the device is obtained by sensing each discrete fluxoid transition in the SQUID, caused by the combined effects of the analog signal current, the correction current, and the high frequency dither signal. The presence or absence of a fluxoid event is interpreted by the control logic, and the feedback register is adjusted accordingly to produce a correction current that maintains the desired operating point for the SQUID.
  • the normal sensitivity of a SQUID quantizer is largely determined by the magnitude of one magnetic flux quantum, given by:
  • the sensitivity is a fraction of this quantum, given by:
  • N is the number of bits of precision in the DAC and in the feedback register.
  • the absolute sensitivity will also be determined in part by the SQUID input inductance and the efficiency of coupling with the input signal. These considerations are similar to those relating to dc SQUID sensors and amplifiers.
  • the sensitivity will also depend on the precision of the DAC, and the precision of the dither amplitude in relation to the current-flux curve. This amplitude directly affects the precision with which the SQUID can be locked to a desired operating point. If the dither amplitude is too large, there will be a dead zone in which the analog signal can change without detection.
  • the total dynamic range of the device is, as has been described, not limited by the number of bits N.
  • the dynamic range is given by:
  • M is the number of bits in the integer register.
  • the sampling rate of the converter is limited by the feedback time constant, which is the total time required for pulse detection, accumulation in the feedback register, DAC operation, and analog settling time.
  • Single flux quantum pulses can be generated in several picoseconds, and a Josephson DAC can cycle in less than half of a nanosecond.
  • the control logic is relatively simple. Accordingly, the total loop time constant should be on the order of 10 -9 second, which permits sampling rates approaching 1 GSps (gigasample per second) and bandwidths up to approximately 500 Mhz.
  • the clock frequency should be around 10 GHz or higher.
  • the digital-to-analog converter may be of any conventional type, but for high performance, Josephson junctions should also be employed in the DAC.
  • FIG. 3 One possible arrangement for a DAC using Josephson junctions is shown in FIG. 3.
  • the DAC consists of multiple stages, indicated at 70.1, 70.2, 70.3 and 70.4, from the least significant to the most significant.
  • the least-significant stage includes a single junction 72, the next stage uses two junctions 74 in series, the next four junctions 76 in series and the most-significant stage uses 2 N junctions 78 in series.
  • the digital quantity to be converted is applied to the various stages, each of which functions as a voltage generator. The voltages are added together at a single adding junction 80, for output from the converter.
  • the present invention represents a significant advance in the field of SQUID analog-to-digital converters.
  • the invention provides a highly sensitive converter without sacrificing dynamic range, and including the ability to handle rapid input slew rates. It will also be appreciated that, although one embodiment of the invention has been described in detail for purposes of illustration, various modifications may be made without departing from the spirit and scope of the invention. Accordingly, the invention is not to be limited except as by the amended claims.

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  • Physics & Mathematics (AREA)
  • Condensed Matter Physics & Semiconductors (AREA)
  • General Physics & Mathematics (AREA)
  • Engineering & Computer Science (AREA)
  • Theoretical Computer Science (AREA)
  • Analogue/Digital Conversion (AREA)
  • Measuring Magnetic Variables (AREA)
  • Superconductor Devices And Manufacturing Methods Thereof (AREA)
US06/797,314 1985-11-12 1985-11-12 Superconducting analog-to-digital converter and digital magnetometer and related method for its use Expired - Lifetime US4672359A (en)

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Application Number Priority Date Filing Date Title
US06/797,314 US4672359A (en) 1985-11-12 1985-11-12 Superconducting analog-to-digital converter and digital magnetometer and related method for its use
DE8686306033T DE3684558D1 (de) 1985-11-12 1986-08-05 Supraleitender ad-wandler und digitales magnetometer und verwandtes verfahren zu dessen gebrauch.
EP86306033A EP0222470B1 (en) 1985-11-12 1986-08-05 Superconducting analog-to-digital converter and digital magnetometer and related method for its use
JP61269477A JPS62117419A (ja) 1985-11-12 1986-11-12 超導電アナログ／デジタル変換器及びアナログ／デジタル変換方法
JP4076526A JP2711043B2 (ja) 1985-11-12 1992-03-31 超電導磁力計及びｓｑｕｉｄを使用して磁場を測定する方法

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US06/797,314 US4672359A (en) 1985-11-12 1985-11-12 Superconducting analog-to-digital converter and digital magnetometer and related method for its use

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US5469057A (en) * 1994-03-08 1995-11-21 University Of New Mexico Method and apparatus for extending the dynamic range of DC-squid measurements using a flux tracking loop
WO1997031273A2 (de) * 1996-02-23 1997-08-28 Forschungszentrum Jülich GmbH MEssEINRICHTUNG ZUR MESSUNG MAGNETISCHER FELDER
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WO2003019214A2 (de) * 2001-08-20 2003-03-06 Stl Systemtechnik Ludwig Gmbh Vorrichtung zum einstellen eines arbeitspunktes eines magnetfeldsensors
US6653962B2 (en) * 2001-10-19 2003-11-25 Deepnarayan Gupta Superconducting dual function digitizer
US6734699B1 (en) 1999-07-14 2004-05-11 Northrop Grumman Corporation Self-clocked complementary logic
US9588191B1 (en) 2008-08-18 2017-03-07 Hypres, Inc. High linearity superconducting radio frequency magnetic field detector
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KR920003885B1 (ko) * 1987-08-25 1992-05-16 스미도모 덴기 고오교오 가부시기가이샤 아날로그/디지탈변환기
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WO1997031273A3 (de) * 1996-02-23 1997-10-02 Forschungszentrum Juelich Gmbh MEßEINRICHTUNG ZUR MESSUNG MAGNETISCHER FELDER
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WO2003019214A2 (de) * 2001-08-20 2003-03-06 Stl Systemtechnik Ludwig Gmbh Vorrichtung zum einstellen eines arbeitspunktes eines magnetfeldsensors
WO2003019214A3 (de) * 2001-08-20 2004-05-27 Stl Systemtechnik Ludwig Gmbh Vorrichtung zum einstellen eines arbeitspunktes eines magnetfeldsensors
US20040207397A1 (en) * 2001-08-20 2004-10-21 Christoph Ludwig Device for adjusting an operating point of a magnetic field sensor
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US9588191B1 (en) 2008-08-18 2017-03-07 Hypres, Inc. High linearity superconducting radio frequency magnetic field detector
US10333049B1 (en) 2008-08-18 2019-06-25 Hypres, Inc. High linearity superconducting radio frequency magnetic field detector
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EP0222470A2 (en) 1987-05-20
DE3684558D1 (de) 1992-04-30
EP0222470A3 (en) 1989-11-08
JPS62117419A (ja) 1987-05-28
JPH05291956A (ja) 1993-11-05
EP0222470B1 (en) 1992-03-25
JP2711043B2 (ja) 1998-02-10

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